Underground Line Locator System With Real Time Kinematic And Global Satellite Positioning

ABSTRACT

A precise line locator is presented that provides precise line location. The locator includes a housing; a wand attached to the housing, the wand including an array of low frequency antennas arranged along the wand, the array of low frequency antennas defining an electromagnetic locate axis of the line locator system; a real-time kinematic (RTK) Global Navigation Satellite (GNSS) antenna attached to the housing; a user interface positioned in the housing; and a processing circuit coupled to the array of low frequency antennas, the RTK GNSS antenna, and the user interface, wherein the underground line locator determines locate data of the underground line based on signals from the array of low frequency antennas and determines a precise position of the underground line locator from the RTK GNSS antenna.

RELATED APPLICATION

This application is a continuation of U.S. Pat. Application No.16/880,595, filed May 21, 2020, which claims priority to U.S.Provisional Application Serial No. 62/851,498, filed on May 22, 2019,each of which are herein incorporated by reference in their entirety.

TECHNICAL FIELD

Embodiments of the present invention are related to underground linelocation and, in particular, to an underground locator system withreal-time kinematic and global satellite positioning.

DISCUSSION OF RELATED ART

The process of locating buried utilities (pipes and cables) using lowfrequency signals is well known and widely adopted as a work practice.Line locating instruments typically include an array of spaced antennasthat receive time-varying magnetic field signals generated by theunderground utility itself. Such signals can be the result of currentscoupled into the underground utility by a separate transmitter or areinherent in the underground utility, for example from power lines. Thearray of spaced antennas receives the magnetic fields, which are oftenat specific frequencies. Processing electronics in the line locatinginstrument determines the relative utility position from the linelocating system, including depth, signal currents and other information.Horizontal position and depth of the underground utility, for example,can then be displayed to the user and, in some systems, recordedrelative to the position of the line locator.

Increasingly, applications for line locating systems are used in mappingutilities. These mappings of underground utilities are desired to be asgeographically accurate as possible. Therefore, there is a need todevelop line location systems with highly accurate positionaldetermination.

SUMMARY

According to some embodiments, precise line locator for precise locationof an underground line is presented. A precise line locator according tosome embodiments includes a housing; a wand attached to the housing, thewand including an array of low frequency antennas arranged along thewand, the array of low frequency antennas defining an electromagneticlocate axis of the line locator system; a real-time kinematic (RTK)Global Navigation Satellite (GNSS) antenna attached to the housing; auser interface positioned in the housing; and a processing circuitcoupled to the array of low frequency antennas, the RTK GNSS antenna,and the user interface, wherein the underground line locator determineslocate data of the underground line based on signals from the array oflow frequency antennas and determines a precise position of theunderground line locator from the RTK GNSS antenna.

A method of precisely determining position of an underground lineincludes locating an underground line at a position with a precise linelocator, the precise line locator having an array of low frequencyantennas arranged along a wand, the array of low frequency antennasdefining an electromagnetic locate axis; placing the precise linelocator in a first orientation; determining and logging line locationdata; placing the precise line locator in a second orientation where areal-time kinematic (RTK) Global Navigation Satellite (GNSS) antenna ispositioned to provide a precise position; and determining and loggingthe precise position with the line location data.

These and other embodiments are discussed below with respect to thefollowing figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a precise underground line locator according to someembodiments.

FIG. 2 illustrates operation of a precise underground line locatorsystem using a precision line locator as illustrated in FIG. 1

FIG. 3 illustrates a block diagram illustrating the circuitry of someembodiments of line locator as illustrated in FIG. 1 .

FIG. 4 illustrates a geometry illustrating correction of the preciselocate position for operation of the precision line locator asillustrated in FIG. 1 .

FIG. 5 illustrates the world magnetic model magnetic field declinationlines.

FIG. 6 illustrates a flow chart for operation of a precise line locatorsystem according to some embodiments of the present invention.

FIGS. 7A and 7B illustrate operational orientation of the precise linelocator as illustrated in FIG. 1 .

FIG. 8 illustrates operation of an inertial measurement unit that can beincluded in some embodiments of line locator as illustrated in FIG. 1 .

FIG. 9 illustrates a user interface illustrating measurement of theRoll, Tilt, and Yaw measured in some embodiments of the precision linelocator as illustrated in FIG. 1 that includes an inertial measurementunit.

These figures along with other embodiments are further discussed below.

DETAILED DESCRIPTION

In the following description, specific details are set forth describingsome embodiments of the present invention. It will be apparent, however,to one skilled in the art that some embodiments may be practiced withoutsome or all of these specific details. The specific embodimentsdisclosed herein are meant to be illustrative but not limiting. Oneskilled in the art may realize other elements that, although notspecifically described here, are within the scope and the spirit of thisdisclosure.

This description illustrates inventive aspects and embodiments shouldnot be taken as limiting--the claims define the protected invention.Various changes may be made without departing from the spirit and scopeof this description and the claims. In some instances, well-knownstructures and techniques have not been shown or described in detail inorder not to obscure the invention.

Recent developments in Satellite Positioning Systems or GlobalNavigation Satellite System (GNSS) facilitate precision pin-pointing toa true grid reference with a position accuracy of only a few cm.Furthermore, Real Time Kinematic (RTK) can be used in conjunction withgeo-spatial information and enhance the position accuracy in real-time -true ‘on-the-fly’ positioning with a horizontal accuracy of 10 cm RMS(the actual accuracy varies over the surface of the globe, 10 cm is atypical accuracy) or less.

Combining Cable Locating Systems with GNSS to produce survey maps iswidely adopted. However, these systems tend to build in inaccuracies.Embodiments according to this disclosure solves a particular problemresulting from the deployment of a GNSS Antenna on a Cable LocatingInstrument. In particular, by adding RTK, the inaccuracies involved withdetermining the position of a located underground line accurately usinga GNSS enabled locator can be alleviated.

FIG. 1 illustrates a precision line locator 100, or Cable LocatingInstrument, according to some embodiments. The embodiment illustrated inFIG. 1 can include a wand structure 102 that houses the line-locationantennas. The line location antennas are arranged along the length ofwand structure 102. Wand structure 102 can house any number ofline-location antennas that are oriented to measure magnetic fieldsemanating from an underground in multiple orthogonal dimensions and maybe oriented along wand structure 102 for measuring the depth of theunderground line. In some embodiments, wand structure 102 may house six(6) antennas. A first set of three antennas are arranged to measuremagnetic fields in three orthogonal directions is set at a firstposition along wand structure 102. A second set of three antennas arearranged to measure magnetic fields in three orthogonal direction and isset at a second position along wand structure 102. Wand structure 102therefore defines a locating axis 114 which extends along a central axisof the first and second set of antennas housed in wand structure 102.With the configured set of antennas, the time-varying magnetic field canbe characterized in three dimensions and the difference in magneticfields at two separate positions along locating axis 114 of wandstructure 102 provides information for determine the depth andorientation of the detected underground line. Typically, the antennashoused in wand structure 102 are designed for low frequency cabledetection, a technique that is well known.

Precision line locator 100 further includes an RTK GNSS high frequencyantenna 108 for satellite decoding. The RTK GNSS Antenna 108 is ahigh-fidelity antenna as the requirement is to perform phase sensitivemeasurements for use in conjunction with RTK systems to determineprecise geographical precision measurements of antenna 108.

FIG. 1 further illustrates that precision line locator 100 includes ahand grip 110 that can be used by a user to hold system 100 as a userpasses precision line locator 100 over an underground line to belocated. Further, precision line locator includes a user interface 104that receives input from and provides data to the user during use. Userinterface 104 is conveniently placed in precision line locator 100 foruse by the user. Circuitry 106 can further be included in precision linelocator 100 to perform operation of line locator 100.

As illustrated in the particular example illustrated in FIG. 1 , ahousing 112 is provided and shaped for convenience and functionality.Circuitry 106 and user interface 104 can be mounted within housing 112.Hand grip 110 may be formed in the shape of housing 112. RTK GNSSantenna 108 and wand structure 102 is mounted to housing 112. Housing112 is conveniently shaped to allow a user to handle precision locator100 while passing locator 100 over an underground line. Housing 112defines a locator forward axis 116 along housing 112 between locatoraxis 114 and RTK GNSS antenna 108.

FIG. 2 illustrates functional operation of a precision line locatingsystem 200 that includes precision line locator 100 according to someembodiments. As illustrated in FIG. 2 , precision line locator 100 ispositioned over a buried line 206, which is buried a certain distanceunder surface 208. Buried line 206 is typically coupled to a transmitter204 that couples an AC signal onto line 206. In some applications, forexample power lines, line 206 may carry a signal from another source. Inmany cases, the AC signal coupled onto line 206 by transmitter 204 canbe of a particular frequency that is detected by precision line locator100. Receiver antennas 202, which are housed in wand structure 102,detect a magnetic field generated by the AC signal on underground line206. Precision line locator 100 can, using signals from antennas 202that result from the magnetic fields generated by line 206, determinethe position of line 206 relative to locator 100.

In accordance with some embodiments, the precise location of RTK GNSSantenna 108, which is connected to precision line locator 100, isdetermined by RTK GNSS antenna 108. Consequently, once the position ofunderground line 206 is located by precision line locator 100, theaccurate position of procession line locator 100 is determined by RTKGNSS antenna 108. Consequently, a precise geographic location of line206 can be determined and recorded. Mapping of line 206 can be performedby determining the precise geographic location of line 206 over a numberof locations over line 206.

Real-time kinematic (RTK) positioning refers to a satellite navigationtechnique sued to enhance precision positioning data at RTK GNSS antenna108. RTK positioning employs a stationary receiver 212 that is incombination with RTK GNSS antenna 108. Each of stationary receiver 212and RTK GNSS receiver 108 are in communications with a plurality ofglobal positioning satellites 210, of which satellites 210-1 through210-N are illustrated.

As is well known, the distance between a receiver such as stationaryreceiver 212 or receiver 108 can be determined by calculating the timeit takes for a signal to reach the receiver from the satellite. Thisdelay can be calculated based on information transmitted in thesatellite signal. The calculation of the distance between the receiverand a number of satellites, and the known locations of the satellites,allows for an accurate location determination of the receiver. However,the accuracy that can be achieved is limited to a meter or more,depending on conditions that can include, for example, travel timesbased on atmospheric conditions or other interference with the signals.

RTK positioning follows the same general concept, but uses the carriersignal from each of satellites 210-1 through 210-N along with stationaryreceiver 212 to provide positional accuracies of 1 cm or less. Inparticular, RTK uses the carrier wave of the satellite signal from eachof satellites 210-1 through 210-N to refine the location of the basestation 212. Base station 212 determines a correction to the positionlocated by conventional methods and by determination based on phaseshifts of the carrier wave and sends the positional correction to RTKGNSS receiver 108. In particular, each of RTK GNSS 108 and base station212 measures a phase difference and RTK GNSS 108 receives the phasedifference measured by base station 212 to compare with the phasedifference determined by its measurement to determine a correction.

Consequently, RTK GNSS antenna 108 is used with a system that usesreal-time kinematic techniques rather than code-based positioning ofstandard global positioning. RTK is a technique that uses carrier-basedranging and provides ranges (and therefore positions) that are orders ofmagnitude more precise than those available through code-basedpositioning.

In practice, the RTK system uses a single base-station receiver 212located at a known location along with a mobile station, which in thisapplication is RTK antenna 108 of precision locating system 100. Basestation 212 rebroadcasts the phase of the carrier that it observes, andRTK antenna 108 compare its own phase measurements with the one receivedfrom the base station. This allows RTK antenna 108 of precision locatingsystem 100 to calculate its relative position with respect to basestation 212 to a high accuracy, in some cases to within millimeters. Theactual location then is accurate to within the accuracy of the locationof base station 212, often to within 1 centimeter ± 1 ppm horizontallyand within 2 centimeters ± 1 ppm vertically. This translates to anaccuracy of ± 1 cm over a kilometer. In some embodiments, base station212 may be one of the public RTK NTRIP (Networked Transport of RadioTechnical Commission for Maritime Services (RTCM) via Internet Protocol)Base Stations. Such accuracy is highly valuable when incorporated inprecision line location system 100 for mapping the location ofunderground utilities.

The position of the RTK GNSS Antenna 108 on locator system 100 can bevery important for operation of precision line locator system 100.Although RTK GNSS antenna 108 can be mounted anywhere on housing 112 ofprecision line locator system 100, for good operation RTK GNSS Antenna108 may be positioned to have a clear uninterrupted view of the sky. IfRTK GNSS Antenna 108 is obstructed, a precision location fix may not bepossible. Consequently, in many embodiments, RTK GNSS Antenna 108 ismounted a distance R from locating axis 114.

Some existing application of GNSS receivers involve mounting thereceiver on a high extension mast that places the receiver above anoperator’s head. However, such an arrangement would not function wellwith a line location system as it is very awkward to handle. Althoughnot previously incorporated into line location systems, RTK GNSSreceivers have been used in system such as surveying equipment. Thosesystems use a high extension mast to ensure the RTK GNSS Antenna isabove head height of any user. Such systems work well but are awkwardand impractical for use in a cable locator system.

Although placement of RTK GNSS antenna 108 at a position on housing 112that is directly aligned with locating axis 114 of wand 102 (position Bindicated in FIG. 1 ), which is in alignment with antennas 202, thatposition may not be ideal. Placing RTK GNSS antenna 108 at positionmarked ‘B’ in FIG. 1 may result in significant obscuration of the skydue to the relative position of the user, an effect referred to as‘human shadowing’ in common parlance. Although impractical, position Bensures that RTK GNSS Antenna 108 is aligned on the same vertical axisas locator’s intrinsic electro-magnetic axis indicated as locate axis114 (defined by the array of low frequency antennas 202), this positionresults in obscuring RTK GNSS antenna 108 that may negatively affect theprecise location function of locator 100, or may even make line locator100 inoperative as a precision line locator.

Consequently, in embodiments according to this disclosure, RTK GNSSantenna 108 is placed in a location on housing 112 where it has a clearview of the sky and is not likely to be shadowed by an operator of linelocator 100 while the operating is handling line locator 100 by grip110. As illustrated in FIG. 1 locate RTK GNSS antenna 108 can, forexample, be connected at position A of housing 112 on precision linelocator 100. Taking the above points into consideration leads to theconclusion that position ‘A’ is a viable option and has only a minorimpact on the overall ergonomics for operating precision line locator100. However, placing the RTK GNSS Antenna 108 at position ‘A’ createsanother intrinsic error which will be discussed further below. Namely,RTK GNSS antenna 108 is off the electromagnetic locator axis 114 ofprecision line locator 100 as defined by antennas 202 mounted in wand102.

FIG. 3 illustrates an example block diagram 300 illustrating circuitry106 of precision line locating system 100 as illustrated in FIG. 1 . Asshown in FIG. 2 , circuitry 106 includes a processing circuit 302.Processing circuit 302 can be any combination of electronics, memory,processors, microcomputers, microcontrollers, or other devices thatreceives, and process data as described below. In particular, processingcircuit 302 may include at least one processor executing instructionsstored in a memory. The memory includes a combination of volatile andnon-volatile memory that stores instructions and data that are executedto precisely locate and map one or more underground lines 206 asdescribed further below. During the mapping, when prompted processingcircuit 302 logs locate information (depth, current, magnetic fieldstrengths, lateral offset, etc.) from locator and logs precise positioninformation from RTK GNSS antenna 108 over one or more positions alongunderground line 206 in order to store a precise mapping of the locationof underground line 206. This mapping may be performed over more thanone underground line to fully map utilities over a geographic area.

As illustrated in FIG. 3 , processing circuit 302 receives data fromuser interface 104. User interface 104 can be any user interface mountedinto housing 112. User interface 104 may, for example, include a displayscreen, which may be a touch-screen, positioned physical buttons,speakers, microphones, and/or other devices that allow processingcircuit 302 to provide information to an operator of precision linelocator 100 and allows the operator to input parameters. Such parameterscan be used to configure operating parameters of precision line locator100 or control display configuration operations, for example. Further,user interface 104 may allow a user to indicate marking of theunderground line and the recording of a precision position as measuredby RTK GNSS antenna 108. Further, user interface 104 may include aninterface to another device for transmission of stored data, updatinginstructions stored in the memory of processing circuit 302, orperforming other functions. In some embodiments, user interface 104 mayinclude a wireless communications interface such as Bluetooth or othercommunications standard to perform upload and download functions ofprecision line locator 100. In some embodiments, physical interfacessuch as, for example, USB interfaces may be utilized to download datafrom or upload data to precision line locator 100. In some embodiments,user interface 104 may include interfaces for wireless connection to alocal area network and/or may include cell service for communicationswith cloud-based services, for example for mapping undergroundutilities.

Processing circuit 302 is also coupled to low frequency antennas circuit308, which includes receiver antennas 202. Low frequency antenna circuit308 can include coil antennas as receiver antennas 202 that are capableof measuring time-varying magnetic fields generated in underground line206 as a result of the transmission of signals onto underground line 206by transmitter 204, or by inherently carried signals in underground line206 (e.g. power line signals). Processing circuit 302 may, in somecases, provide digital signals to control the configuration of antennas308. Low frequency antenna circuit 308 includes circuits for receivingsignals from receiver antennas 202 and providing digitized receiversignals to processing circuit 302. For example, antenna circuit 308includes analog filtering and analog-to-digital converters that areconfigured to provide the digital signals. Antenna circuitry 308 thenprovides digitized signals indicating the magnetic field strengths fromeach of receiver antennas 202 to processing circuit 302.

Support circuitry 304 may include any circuitry that is further usedwith locator 100, for example power control circuitry or anyanalog-to-digital or digital-to-analog circuits, filtering of analogsignals, or other actions.

In some embodiments, processing circuit 302 is coupled to an InertialMeasurement Unit (IMU) 306. IMU 306 may contain combinations ofaccelerometers, gyroscopes, and/or magnetometers that allow formeasurement of acceleration and orientation of precision line locator100. In general, IMU 306 may include any number of accelerometers,gyroscopes, and/or magnetometers to measure acceleration with respect toa set of axes with respect to precision line locator 100. For example,IMU 306 may include three accelerometers positioned to measureacceleration along three orthogonal axes and three gyroscopes positionedmeasure angular acceleration around each of the three, orthogonal axes.In some embodiments, IMU 306 may include magnetometers that measure themagnetic fields along the three orthogonal axes. The acceleration datafrom IMU 306 is digitally provided to processing circuit 302, which candetermine the current orientation of precision position sensor 100. Insome embodiments, the orientation may be determined with respect tolocate axis 114 and locator axis 116.

Processing circuit 302 is also coupled to RTK GNSS antenna 108. RTK GNSSantenna 108 includes antennas and receive circuits for receivingsatellite signals from satellites 210 and antennas and receive circuitsfor receive phase data from base station 212. In some embodiments, basestation 212 and RTK GNSS antenna 108 can communicate using UHFsignaling. However, any communications technique can be used to providedata to RTK GNSS antenna 108 from base station 212.

RTK techniques may involve complex calculation based on the receivedsignals from satellites 210 and the phase data from base station 212. Insome embodiments, RTK GNSS antenna 108 includes detection and processingcircuits that determine the precise location of RTK GNSS antenna 108according to the satellite signals as discussed above. In that case, RTKGNSS antenna 108 provides precise positional data to processing circuit302. In some embodiments, RTK GNSS antenna can provide the receivedsignals from satellites 210 and the phase data from base station 212 toprocessing circuit 302, where the calculation of precise location of RTKGNSS antenna 108 is calculated.

FIG. 4 illustrates the geometry of precise location measurement with thespecific example of precision line locator system 100 presented in FIG.1 . FIG. 4 illustrates an orientation of precision line locator 100 withrespect to true north N and positioned such that wand 102, which definesthe magnetic axis, positioned vertically. In the example illustrated inFIG. 4 , precision line locator 100 is held such that locator axis 114is held vertically, usually over an underground line that has beenlocated so that a precise location of that line can be obtained usingRTK GNSS antenna 108.

As illustrated in FIGS. 1 and 4 , the value R represents the separationof the locator axis 114, also referred to as the magnetic axis, to theposition of the RTK GNSS antenna 108. The angle θ is the angle betweenthe locator forward axis 116 and true North. In this example, θ is360° - the bearing (e.g. the forward direction along locator forwardaxis 116). As discussed above, the locating axis 114 is defined by thelow frequency antenna geometry.

As illustrated in FIG. 4 , the distance R between the RTK GNSS Antennaaxis and the Locating Axis 114 creates an error relative to the truegrid reference, which can be at any angle with respect to orientation ofprecision line locator 100. As is illustrated in FIG. 4 , the error inlatitude and the error in longitude is defined with respect to thedirection of true north N and the position of RTK GNSS 108. Withlocating axis 114 positioned vertically over a spot where a preciselocation is to be determined, the distance between RTK GNSS 108 being adistance R from locating axis 114 along locator forward axis 116, andthe angle Θ between true north N and locator axis Θ, the error in truelatitude and longitude can be calculated as follows:

Error in Latitude = R cosine(360 − θ);

Error in Longitude = R sine(360 − θ).

One solution to correct the above errors is to measure the angle θ andapply the corrections using software executed in processing circuitry302. Such a solution assumes an accurate and reliable method ofestablishing a true grid angular reference. In some embodiments, IMU 306can include a flux-gate magnetometer that can be used to measure theearth’s magnetic field which is generally within a few degrees of truenorth (angular reference). This method can work well but cannot beregarded as reliable.

One problem is the changes in magnetic declination. The magneticdeclination refers to the angle between true north N and the magneticnorth, which is the local direction of the earth’s magnetic field. Themagnetic declination varies significantly over the surface of the earthas is illustrated in FIG. 5 . FIG. 5 illustrates a Mercator mapillustrating magnetic declination isogonic lines developed by theNational Oceanic and Atmospheric Administration (NOAA) with the NationalGeophysical Data Center (NGDC) and the Cooperative Institute forResearch in Environmental Sciences (CIRES). Contour line intervals are 2degrees where lines marked “red” represent positive declinations(eastward), “blue” represents negative declinations (westward) and“green” are zero declination lines.

Further, local variations caused by parked vehicles and other ironstructures can cause an angular measurement error of up to 180°.Embodiments of the present invention provide a simple method to solvethe above problems allowing line locator system 100 to combine accurateutility location with cm accurate geo-spatial data using RTK with RTKGNSS antenna 108.

FIG. 6 illustrates a method 600 of operating an embodiment of preciseline locator 100 to log and store data regarding the position of anunderground line 206 over a geographic area to form a map. Someembodiments as described here present a simple but accurate method thatcorrects the intrinsic errors described above. As discussed above, thelocate position is defined with respect to the electro-magnetic locateaxis 114, which is defined by antennas 202 in wand 102 of precision linelocator 100. FIGS. 7A and 7B illustrate how precision line locator 100can be operated during various steps of method 600 in order to firstlocate and log location data regarding underground line 206 with respectto precision line locator 100 and then to reorient precision linelocator 100 to obtain and log a precise position of line locator 100with RTK GNSS antenna 108.

As indicated in FIG. 6 , in step 602 precision line locator 100 isoperated as a line locator used to locate the position of undergroundline 206 using antennas 202. Line locator 100 may precisely locate theposition of underground line 206 relative to precision line locator 100.Once underground line 206 is located with precision line locator 100,line locator 100 is placed in a first orientation 704 over undergroundline 206 in step 604.

FIG. 7A illustrates orientation 704 of step 604, where bottom 702 isplaced on ground surface 208 and locate axis 114 is positioned to bevertical. Orientation 704 is used to provide accurate location ofunderground line 206 by precision line locator 100 using antennas 202 inwand 102. In operation, users place a bottom 702 of wand 102 at groundlevel 208 and align locator axis 114 vertically to eliminate any offseterrors in the determination of the position of underground line 206 withrespect to precision line locator 100.

Once the locate position is defined in orientation 704, in step 606locate information is logged. Typically, the logging at this stagecauses an array of electro-magnetic measurements to be recorded inmemory of processing circuit 302. These measurements may include, forexample, the depth of line 206, the current through line 206 as measuredby locator 100, a lateral offset indicating that locate axis 114 is notdirectly over underground line 206, and fault measurements of theutility relative to the line locating system 100. Logging in step 606may occur when precise line locator 100 determines that it is inorientation 704 or when prompted on user interface 104 by the user.

Once the locate information is logged in step 606, in step 608 precisionline locator 100 prompts the user to position locator 100 forrecordation of precise position data. FIG. 7B illustrates an orientation706 that allows for precise measurement of the position of undergroundline 206 using RTK GNSS antenna 108. In orientation 706, the user isprompted to tilt precision line locator 100 backwards as indicated inFIG. 7B until RTK GNSS antenna 108 is aligned with vertical axis 708,which extends vertically through bottom 702. At the point the RTK GNSSAntenna 108 is directly over the locate position, the RTK geo-spatialposition can be measured and appended to the measurement log with thelocate measurement data discussed above in step 610.

In orientation 706, RTK GNSS antenna 108 and the defined locate positionare now in the same vertical line, the error caused by the offsetdisplacement R is eliminated. Consequently, the value R in the abovecorrection equations can be set to 0 in the correction equationsdescribed above. In other words, RTK GNSS 108 has been brought into linewith where locator axis 114 was during the locate operation andconsequently the value of R is reduced to 0 in the correctioncalculation.

In step 612, method 600 determines whether or not data is to be loggedfor any more positions. If not, then precision line locator 600 isstopped in step 614. If so, then precision line locator 600 is moved tothe next position in step 616 and method 600 returns to step 602. Inthat fashion, a mapping of locate data and precise position data for anumber (1 or more) of positions is stored in the memory of processingcircuit 302. Each logged data can be used to determine the preciseposition of underground line 206 at each of these positions because thegeometry of precise line locator 100 itself is well defined. Calculationof the precise position of underground line 206 at each position can beperformed from the log data at a later time, or in some embodiments maybe performed by processing circuit 302.

In some embodiments, an inertial measurement unit (IMU) 306 may beincluded. IMU 306 may be a useful feature to provide guidance inpositioning precision line locator 100 in position 704 of FIGS. 7A or706 of FIG. 7B. In some examples, IMU 306 may include combined MEMs gyroand accelerometer. IMU 306, under control of processing circuit 302,yields real-time measurements of roll, pitch and yaw as illustrated inFIG. 8 . In some embodiments, the update rate is typically 26 Hz (100 Hzmax) and the measurement accuracy is typically within ± 1°.

As illustrated in FIG. 8 , IMU 306 inertially measures accelerationalong locate axis 114 (the Z direction), forward locate axis 116 (the Xdirection), and the Y direction perpendicular to the Z direction and theX direction. IMU 306 further measures angular rotational accelerationaround each of axis X, Y, and Z. IMU 306 further includes gyros thatmeasure rotational acceleration around the X axis (Roll Φ), rotationalacceleration around the Y axis (Pitch θ), and rotational accelerationaround the Z axis (Yaw ψ). Processing circuitry 302 receives theacceleration data from IMU 306 and can calculate the orientation ofprecise line locator 100.

In some embodiments, the measured angles may be referenced to thelocator user interface 104 display, which is featured into the handle atthe top of the locator as indicated in FIG. 1 . It is therefore possibleto sense the exact point in which the locate position and the RTK-GNSSAntennas are vertically collinear. The actual offset (mainly the pitchangle) is defined by the physical dimensions of precise locator 100 andis both fixed and known.

In some embodiments, guidance in the alignment process can be providedon user interface 104 as illustrated in FIG. 9 . As is illustrated inFIG. 9 , user interface 104 includes a display 902 that can show theangle compensation according to user settings (off, dynamic, fixed).Display 902 is currently illustrating the dynamic user setting. Whenangle compensation is “Dynamic” roll and pitch are updated according tothe live measurements from IMU 306. FIG. 9 shows the pitch and roll ascalculated from 3D accelerometers (XL) in indicator 904, 3D gyroscopes(G) as indicated in indicator 906 and the combined XL & G pitch and rollvalues as indicated in indicator 908, as described above. Further, aspirit level 910 demonstrates alignment of the locator axis 114 withrespect to gravity. Further, user interface 104 can include user inputbuttons 912 to control operation of precise locator 100. In someembodiments, spirit level 910 may be computer generated or a mechanicallevel. Spirit level 910 may be arranged to indicate positioning inorientation 704 of FIG. 7A and later positioning in orientation 706illustrated in FIG. 7B.

Although user interface 104 may be arranged or distributed differentlyas illustrated in FIG. 9 , some embodiments can include the‘spirit-level’ widget 910 to alert the user that the exact RTK positionof alignment has been reached. This alert also causes the exact positionto be measured allowing this data to be appended to the standard surveymap data as discussed above. In some embodiments, spirit-level widget910 may display a first level for alignment in orientation 704 asillustrated in FIG. 7A and display a second level for alignment inorientation 706 as illustrated in FIG. 7B.

In some embodiments, it may be found that it is not necessary to movethe locator to the exact point of collinearity. Given that the 3inertial measurements of roll, pitch and yaw are continuously updatingand that there is a known trigonometric relation between the locator’selectro-magnetic axis and the RTK Antenna it can be reasoned that asmaller displacement is adequate. Displacements from ideal positioningcan be corrected by calculation in processing circuit 302 based on datafrom IMU 306.

The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous variations and modifications within the scope of the presentinvention are possible. The present invention is set forth in thefollowing claims.

What is claimed is:
 1. A precise line locator, comprising: a housing; awand attached to the housing, the wand including an array of lowfrequency antennas arranged along the wand, the array of low frequencyantennas defining an electromagnetic locate axis of the line locatorsystem; a real-time kinematic (RTK) Global Navigation Satellite (GNSS)antenna attached to the housing; a user interface positioned in thehousing; and a processing circuit coupled to the array of low frequencyantennas, the RTK GNSS antenna, and the user interface, wherein theunderground line locator determines locate data of the underground linebased on signals from the array of low frequency antennas and determinesa precise position of the underground line locator from the RTK GNSSantenna.
 2. The precise line locator of claim 1, wherein the processingcircuit logs the locate data and the precise position one or moreposition points of the underground line.
 3. The precise line locator ofclaim 1, wherein the housing defines a forward locate axis that isperpendicular to the electromagnetic locate axis and along which a userhandgrip is formed and the user interface and the RTK GNSS antenna areattached, the RTK GNSS antenna being separate from the electromagneticlocate axis.
 4. The precise line locator of claim 3, wherein the locatedata is determined with the precise line locator in a first orientationand where the precise position is determined with the precise linelocator in a second orientation.
 5. The precise line locator of claim 4,wherein the first orientation is achieved with the electromagneticlocate axis aligned with a vertical line and with a bottom of the wandresting on a ground over the underground line.
 6. The precise linelocator of claim 5, wherein the second orientation is achieved with thebottom of the wand remaining on the ground over the underground line andthe RTK GNSS antenna positioned along the vertical line.
 7. The preciseline locator of claim 4, wherein the processing circuit executesinstructions stored in a memory to locate the underground line with thearray of low frequency antennas; determine and log locate data when theline locator is in the first orientation; and determine and log theprecise position with the locate data.
 8. The precise line locator ofclaim 4, further including an inertial measurement unit.
 9. The preciseline locator of claim 8, wherein data from the inertia measurement unitis displayed on the user interface to assist placement of the linelocator in the first orientation and the second orientation.
 10. Theprecise line locator of claim 9, further including one or more spiritlevel displays to assist placement of the line locator in the firstorientation and/or the second orientation.
 11. A method of preciselydetermining position of an underground line, comprising: locating anunderground line at a position with a precise line locator, the preciseline locator having an array of low frequency antennas arranged along awand, the array of low frequency antennas defining an electromagneticlocate axis; placing the precise line locator in a first orientation;determining and logging line location data; placing the precise linelocator in a second orientation where a real-time kinematic (RTK) GlobalNavigation Satellite (GNSS) antenna is positioned to provide a preciseposition; and determining and logging the precise position with the linelocation data.
 12. The method of claim 11, further including producing amap of the underground line by logging the line location precisionposition at a plurality of positions.
 13. The method of claim 11,wherein the wand is attached to a housing that defines a forward locateaxis that is perpendicular to the electromagnetic locate axis and alongwhich a user handgrip is formed and a user interface and the RTK GNSSantenna are attached, the RTK GNSS antenna being separate from theelectromagnetic locate axis.
 14. The method of claim 13, wherein thefirst orientation is achieved with the electromagnetic locate axisaligned with a vertical line and with a bottom of the wand resting on aground over the underground line.
 15. The method of claim 14, whereinthe second orientation is achieved with the bottom of the wand remainingon the ground over the underground line and the RTK GNSS antennapositioned along the vertical line.
 16. The method of claim 13, whereinplacing the precise line locator in the first orientation includesindicating the first orientation on the user interface based on datareceived from an inertial measurement unit.
 17. The method of claim 13,wherein placing the precise line locator in the second orientationincludes indicating the second orientation on the user interface basedon data received from an inertial measurement unit.
 18. The method ofclaim 13, wherein placing the precise line locator in the firstorientation includes indicating the first orientation on the userinterface based on data received from a spirit level.
 19. The method ofclaim 13, wherein placing the precise line locator in the secondorientation includes indicating the second orientation on the userinterface based on data received from a spirit level.